1. Field of the Invention
This invention relates to a transflective type LCD device, and more particularly, to a transflective type LCD device that has a transmissive area which transmits light from the rear surface side to the display surface side to display an image, and a reflective area which reflects light incident from the display surface side to display an image.
2. Description of the Related Art
LCD devices are generally classified into transmissive type LCD devices and reflective type LCD devices. In general, a transmissive type LCD device has a backlight source, and controls the transmission amount of light from the backlight source to thereby display an image. A reflective type LCD device has a reflection film which reflects light from the outside, and utilizes light reflected by the reflection film as a display light source to thereby display an image. The reflective type LCD device, which does not require a backlight source, is superior in reduction of the power consumption, for a smaller thickness and a lower weight, as compared with the transmissive type LCD device. However, since the ambient light is used as a display light source, there is a defect that, when the ambient area is dark, the visibility is lowered.
As an LCD device which has the advantage of the transmissive type LCD device and that of the reflective type LCD device, there is known a transflective type LCD device (for example, refer to Patent Publication JP-2003-344837A. The transflective type LCD device has a transmissive area and a reflective area in each pixel. The transmissive area transmits light from a backlight source, and sets the backlight source as a display light source. The reflective area has a reflection film, and the light incident from the outside and reflected by the reflection film is used as a display light source. In using the transflective type LCD device, in case the ambient area is bright, the backlight source is turned off, and an image is displayed on the screen by the reflective area, which can realize reduction of the power consumption. On the other hand, in case the ambient area is dark, the backlight source is turned on, and an image is displayed on the screen by the transmissive area, which can display the image even if the ambient area is dark.
As the display mode of the LCD device, there are an IPS mode (In-plane-Switching mode) and an FFS mode (Fringe-Field-Switching mode) which are the lateral-direction-electric-field mode excellent in the contrast of transmission and the viewing angle thereof. The LCD device of the lateral-direction-electric-field mode such as the IPS mode and FFS mode has a pixel electrode and a common electrode which are formed on the same substrate, and applies an electric field of the lateral direction to an LC layer. Due to the configuration wherein the LCD device of the lateral-direction-electric-field mode displays an image by rotating LCD molecules in a direction parallel to the substrate, a higher viewing angle can be realized in the lateral-direction-electric-field mode, as compared with an LCD device of the TN mode.
However, in case the transflective type LCD device employs the lateral-direction-electric-field mode such as the IPS mode and FFS mode, as is described in JP-2003-344837A there is raised a problem that display of dark state (black) and display of bright state (white) are inverted. In the usual drive system, when the transmissive area is set to normally black, the reflective area assumes normally white. Hereinafter, the reason of the inverted display will be described. FIG. 20A shows a schematic view indicative of a section of the transflective type LCD device, and FIG. 20B shows a schematic view indicative of the polarized state of light of respective areas when the light advances from a polarizing film, through an LCD layer, and to a polarizing film. An arrow represents that the polarized state of light is the linear polarization, an encircled R represents that the polarized state is the clockwise circular polarization, and an encircled L represents that the polarized state is the counterclockwise circular polarization. A round bar represents a director (molecule) of LC.
Each of pixels of an LCD device 50 has a reflective area 55 and a transmissive area 56. The reflective area 55 sets reflected light from a reflection film 54 to a display light source, and the transmissive area 56 sets a backlight source, not shown, to a display light source. A polarizing film (first polarizing film) 51 on the viewer side, or front side, and a polarizing film (second polarizing film) 52 on the rear side are arranged such that the polarizing axes thereof are perpendicular to each other. In an LC layer 53, LC molecules are arranged such that the direction of LC molecules upon absence of applied voltage is deviated from the polarizing axis (light transmission axis) of the second polarizing film 52 by 90 degrees. For example, when the polarizing axis of the second polarizing film 52 is at 0 degree, the polarizing axis of the first polarizing film 51 is set to 90 degrees, and the longer axis direction of LC molecules of the LC layer 53 is set to 90 degrees. In the LC layer 53, the cell gap is adjusted such that the retardation Δn·d (Δn represents the refractive index anisotropy of LC molecules, and “d” represents the cell gap of LC layer) assumes λ/2 (λ is the wavelength of light, and, for example, if green light is selected as the standard light, λ=550 nm) in the transmissive area 56, while the cell gap is adjusted such that the retardation assumes λ/4 in the reflective area 55.
Firstly, the operation upon absence of applied voltage on the LC layer 53 will be described.
<Reflective Area, Upon Absence of Applied Voltage>
The reflective area upon absence of applied voltage will be described.
In the reflective area 55, linearly polarized light of 90 degrees direction (longitudinal direction) passing through the first polarizing film 51 advances to the LC layer 53. In the LC layer 53, since the optical axis of the linearly polarized light travelling to the LC layer matches the longer axis direction of LC molecules, the light passes through the LC layer 53 with its polarized state being kept at linearly polarized angle of 90 degrees, and is reflected by the reflection film 54. In case of the linearly polarized light, since the light is kept linearly polarized after being reflected, the light advances to the LC layer 53 again with its polarized state being kept at linearly polarized angle of 90 degrees. Furthermore, while the light advances from the LC layer 53 to be incident onto the first polarizing film 51 with its polarized state being kept at linearly polarized angle of 90 degrees, since the polarizing axis of the first polarizing film 51 is also at 90 degrees, the light passes through the first polarizing film 51. Accordingly, upon absence of applied voltage, the display represents a bright state or black.
<Reflective Area, Upon Presence of Applied Voltage>
The reflective area upon presence of applied voltage will be described.
In the reflective area 55, linearly polarized light of 90 degrees direction (longitudinal direction) passing through the first polarizing film 51 advances to the LC layer 53. Upon presence of applied voltage on the LC layer 53, the longer axis direction of LC molecules in the LC layer 53 is changed from 0 degree to 45 degrees on the substrate surface. In the LC layer 53, since the polarized direction of the incident light is deviated from the longer axis direction of LC molecules by 45 degrees, and the retardation of the LC is set to λ/4, linearly polarized light of the longitudinal direction, which advances to the LC layer 53, advances to the reflection film 54 with its polarized state being set clockwise-circularly polarized. This clockwise-circularly polarized light is reflected by the reflection film 54 and has its polarized state being set counterclockwise-circularly polarized. The counterclockwise-circularly polarized light, which advances to the LC layer 53, passes through the LC layer 53 again, and has its polarized state being set to linearly polarized state of the lateral direction (0 degree direction) to advance to the first poling film 51. Since the polarizing axis of the first polarizing film 51 is at 90 degrees, the light reflected by the reflection film 54 cannot be made to pass through, and the display represents a dark state.
As described above, in the reflective area, the display assumes the normally white display, in which the display represents a bright state upon absence of applied voltage, while the display represents a dark state upon presence of applied voltage.
<Transmissive Area, Upon Absence of Applied Voltage>
Next, the transmissive area will be described. Firstly, the state upon absence of applied voltage will be described.
In the transmissive area 56, linearly polarized light of the lateral direction passing through the second polarizing film 52 advances to the LC layer 53. In the LC layer 53, since the polarized direction of the incident light is perpendicular to the longer axis direction of LC molecules, without changing the polarized state, the light passes through the LC layer 53 with its polarized state kept linearly polarized of the lateral direction, and advances to the first polarizing film 51. Since the polarizing axis of the first polarizing film 51 is at 90 degrees, the transmitted light cannot pass through the first polarizing film 51 and the display represents a dark state.
<Transmissive Area, Upon Presence of Applied Voltage>
Next, the state upon presence of applied voltage will be described. In the transmissive area 56, linearly polarized light of the lateral direction passing through the second polarizing film 52 advances to the LC layer 53. Upon presence of applied voltage on the LC layer 53, the longer axis direction of LC molecules in the LC layer 53 is changed from 0 degree to 45 degrees on the substrate surface. In the LC layer 53, since the polarized direction of the incident light is deviated from the longer axis direction of LC molecules by 45 degrees, and the retardation of the LC is set to λ/2, linearly polarized light of the lateral direction, which advances to the LC layer 53, advances to the first polarizing film 51 with its polarized state being set to linearly polarized state of the longitudinal direction. Accordingly, in the transmissive area 56, the first polarizing film 51 allows the backlight incident onto the second polarizing film 52 to pass therethrough, and the display represents a bright state or white.
As described above, in the transmissive area, the display assumes a normally black mode, in which the display represents a dark state upon absence of applied voltage, while the display represents a bright state upon presence of applied voltage.
As a method to solve above-described problems, JP-2006-180200A describes a device configuration for solving the problem of the display inversion between the transmissive area and the reflective area, while using a specific signal processing and driving technique for the LCD device. The LCD device described in JP-2006-180200A is a transflective type LCD device including a pair of polarizing films which have an LC layer sandwiched therebetween. The polarizing films have polarizing axes which are perpendicular to each other. Each pixel of the LCD device includes a transmissive area and a reflective area and is driven by the lateral-electric-field mode, wherein the longer axis of LC molecules in the LC layer is parallel or perpendicular to the polarized direction of light which advances to the LC layer in the transmissive area. Each pixel has a pixel electrode arranged in a transmissive area and a reflective area of the pixel which is driven by a common data signal, a first common electrode to which a first common signal which is shared by reflective areas of a plurality of pixels is applied, and a second common electrode to which a second common signal which is shared by transmissive areas of the plurality of pixels is applied.
FIG. 21 shows a schematic view indicative of the planar configuration in a single pixel of the LCD device described in JP-2006-180200A. An LCD device 100 includes a first common electrode 137 which corresponds to a reflective area 121, a second common electrode 138 which corresponds to a transmissive area 122, and a pixel electrode 135 which supplies a common data signal to the reflective area 121 and transmissive area 122. In the reflective area 121, the LC layer is driven by an electric field generated by the pixel electrode 135 and the first common electrode 137, and in the transmissive area 122, the LC layer is driven by the electric field generated by the pixel electrode 135 and the second common electrode 138. In this configuration, since a signal (electric potential) applied to the first common electrode 137 and a signal applied to the second common electrode 138 are controlled such that the magnitude of electric field applied to the LC layer in the reflective area 121 and the magnitude of electric field applied to the LC layer in the transmissive area 122 are opposite to each other, the display in the reflective area and the display in the transmissive area have the same display mode. Accordingly, the problem of the transflective type LCD device, or the problem of the inversion of display of a bright/dark state between the reflective area and the transmissive area can be solved.
Specifically, a first common signal and a second common signal supplied to the first common electrode 137 and the second common electrode 138, respectively, are inverted in synchrony with a pixel signal supplied to the pixel electrode 135, wherein the first common signal is obtained by substantially inverting the second common signal. In this case, for example, when an electric potential of 5 V is applied to the pixel electrode 135 in the reflective area 121 and transmissive area 122, by setting the first common electrode 137 to 0 V, and setting the second common electrode 138 to 5 V, the LC layer can be rotated only in the reflective area 121, and the problem of the inversion of display of bright state and display of dark state between the reflective area 121 and the transmissive area 122 can be solved. In employing this configuration, it is not necessary that the first common signal and the second common signal have to be inverted signals in a strict sense. For example, the first common signal may assume 0 V or 5 V, and the second common signal may to assume 6 V or 0 V. Hereinafter, the drive system for LCD device in JP-2006-180200A is referred to as an inverting drive system using an inverting drive scheme, for the sake of convenience.
On the other hand, in the transflective type LCD device, in order to allow the image quality in the reflective mode to match the image quality in the transmissive mode, it is important that the voltage-luminance characteristics including VR (voltage-reflectance) characteristics and VT (voltage-transmittance) characteristics in the reflective area matches those in the transmissive area. For example, in a literature entitled “A single Gap Transflective Fringe-Field Switching Display”, SID2006 P159 (p. 810), there is described an LCD device that performs the FFS drive mode with the same gap setup in a reflective area and in a transmissive area. In this technique, the LCD is of the transflective type and uses the lateral-electric-field mode without using an inverting drive system, wherein an in-cell retarder is used only in the reflective area, to optically solve the problem of the inversion between the reflective area and the transmissive area, and then, the VR characteristics and VT characteristics are allowed to match between the reflective area and the transmissive area. The technique solving the problem is such that the transmissive area is driven using the FFS-mode drive, and the reflective area is driven using the IPS-mode drive, and the angle formed between electrodes in the form of comb teeth and the rubbing angle in the transmissive area is set to approximately 80 degrees, and the angle formed between electrodes in the form of comb teeth and the rubbing angle in the reflective area is set to approximately 45 degrees, which makes the VT/VR characteristics match between both the areas. This technique compensates the difference between both the drive voltages, which occurs due to the same cell gap provided in the reflective area and the transmissive area.
In the configuration described in the JP-2006-180200A, both the VT characteristics and VR characteristics are opposite to each other. That is, the VT characteristics is such that a higher voltage provides a higher transmittance whereas the VR characteristics is such that a higher voltage provides a lower reflectance, thereby raising a problem that the image quality in the reflective mode does not match the image quality in the transmissive mode. A method to solve the problem of the image quality in the inverting drive scheme is not known. It is recited in the above literature that, with respect to the problem that the VR characteristic and VT characteristic have a deviation therebetween due to the same cell gap being provided in the reflective mode and transmissive mode, only the angle formed between electrodes in the form of comb teeth and the rubbing angle has a difference between the reflective area and the transmissive area That is, this technique is silent to the solution for allowing the image quality in the reflective mode to match the image quality in the transmissive mode.